In the energy industry, the Coal Bed Methane Process (CBMP) has been used to recover methane gas from water pumped to the surface from underground coal beds. This water pumped to the surface generally must be treated before returning it to the environment due to contaminants deemed unsuitable for the environment or other uses. This clean-up process may involve the use of an ion exchange apparatus.
Methods for ion exchange of CBMP water can include simple batch ion exchange and/or continuous ion exchange processes such as the Higgins loop. Equipment for a continuous or simulated continuous process is generally more expensive to install and often considered more complicated to operate than a batch process. However, continuous processes are more efficient than simple batch processes so it is understood that they are presently favored over batch processes in spite of capital costs and operating complexities generally associated with them. It is understood that a particular problem for a batch process compared to the continuous processes is that the quantity of regenerant waste stream produced by a batch process is of a much greater volume and is more dilute than continuous processes. Because the regenerant waste often must be transported and discarded in some procedure to comply with environmental regulation, the greater volume of waste produced can make the batch process uneconomical in a typical commercial operation.
A conventional process of batch ion exchange can be described as follows, as will be understood by one of ordinary skill in the art. In this example, the water treated from a CBMP is assumed to contain sodium bicarbonate as the primary contaminant to be removed and reduced in concentration. Also in this example, an acid is used in the process for regeneration which is assumed to be hydrochloric acid. Other regenerates may be considered, for example only, as sulfuric acid, although for acids hydrochloric acid is presently common if the process under consideration is CBMP. As understood by those of ordinary skill in the art, other contaminants, acids, or parameters may be considered relevant. As an example, such process may include:
1. A column is filled with a strong or weak cation resin bed in the hydrogen form, depending on the circumstances.
2. Water from the CBMP is introduced to the column and is passed downflow at some appropriate rate, e.g., 5 to 20 bed volumes/hour. Preferably, the column should be pressurized to avoid evolution of carbon dioxide gas in the column.
3. The resin exchanges the hydrogen in the resin for the cations in the water. Typically sodium makes up the majority of the cations present in the water from the CBMP prior introduction to the column.
4. The water exiting the column contains carbonic acid, which can be decomposed and neutralized to yield carbon dioxide and purified water.
5. The column of resin which is now in the sodium form is regenerated with hydrochloric acid. This regeneration is accomplished by passing a given quantity of the acid through the resin which exchanges hydrogen from the acid for the sodium. The hydrochloric acid which typically is commercially available in a concentrated form is first diluted with water to a typical concentration of 12.5%. The acid is rinsed off the resin using water.
6. The regenerant waste, typically called “brine”, exits the column as a profile. The exchanged hydrochloric acid results in a waste containing primarily sodium chloride. This waste increases in concentration of dissolved solids from when the waste exits the column and eventually the profile decreases to essentially zero concentration on the tail end as the final regenerate waste is rinsed from the column.
In commercial operation it is understood that a batch process is considered fairly inefficient compared to continuous processes. It produces a greater volume of waste, which also is a much more dilute waste than in a continuous process. These forgoing distinctions can be considered important efficiency factors in commercial operation.
Note that the amount of hydrochloric acid to be used for regeneration in the batch process is not calculated one to one on the sodium to be stripped from the resin. In order to regenerate the column of resin properly, an excess amount of acid must be added to the column. As a consequence, there is a “leakage” of excess acid on the tail end of the regenerant waste profile. To improve the batch operation somewhat, it is understood to be conventional practice to save the highest concentration acid leakage from the tail end of the regenerant waste profile and use it prior to the next regeneration to obtain a small amount of additional exchange efficiency. This practice is typically called recycle of waste regenerant. The purpose is to take advantage of the residual leaked acid potential in the waste, to give it a second chance to regenerate the resin.
The leading front of the exiting regeneration waste profile is also problematic with the batch process. It is diluted with water and spread out over a profile compared with continuous methods. This dilution results in a dilution of the regenerant waste (brine) and a large volume of regenerant waste.
The overall large volume of waste with low salt (e.g., brine) concentration associated with the batch ion exchange process makes the batch process unacceptable for treatment of water in the CBMP industry, and to our understanding has been purposely avoided for such use.